The present invention relates to a method for applying a liquid solution to a running web for use in producing photographic films, photographic printing paper, etc., (hereinafter referred to collectively as "photographic light-sensitive elements"), and particularly relates to a multi-layer simultaneous coating method.
In producing photographic light-sensitive elements, generally, emulsions of a so-called oil-in-water dispersion type are used. In producing such an emulsion, sometimes a low boiling point solvent such as ethyl acetate, butanol, or the like is used as an auxiliary solvent. When a liquid coating containing such a low boiling point solvent is used to form the outermost layer in a multi-layer simultaneous coating method, it is difficult to obtain a stable and uniform film coating. This is because if such a low boiling point solvent is contained even in small amounts in the outermost layer, the solvent in the liquid surface can easily be evaporated by contacting only a very weak flow of air. The nonuniform distribution of surface tension in the free surface of the coating composition caused thereby produces disorder in the liquid films and hence unevenness of coating.
To prevent such uneven coating from occurring, the following methods have been proposed:
(1) a method whereby the content of an organic solvent contained in the coating composition is not more than 5 wt % (see, for example, Japanese Unexamined Patent Publication No. Hei. 3-92846); and
(2) a method whereby the content of a solvent in the outermost layer is not more than 1 wt %, or a method using an apparatus for weakening the air flow over the coating portion by use of an air shield (see, for example, Japanese Patent Application No. Hei. 1-320640).
However, in the case where the distance from the outermost vapor-liquid surface to the silver halide containing layer is small, or in the case where a large quantity of a low boiling point solvent is contained in the layer next to the innermost layer, it is not sufficient to use only the above-mentioned stabilizing method, even if the content of low boiling point solvent in the outermost layer is not more than 1 wt %. Uneven coating still results because of the uneven surface tension distribution caused by nonuniform evaporation on the liquid surface of the low boiling point solvent. Therefore, in the case where the silver halide containing layer is close to the liquid surface, when any thickness unevenness in coating occurs, even if it is slight, the thickness of the silver halide containing layer will also be nonuniform.
If the distance between the liquid surface and the silver halide containing layer is lengthened, no unevenness in coating occurs, even if the content of the low boiling point solvent in the outermost layer is about 1 wt %. In the case where a larger quantity of low boiling point solvent is contained in the inner layer next to the outermost layer, on the other hand, the low boiling point solvent contained in the inner layer will diffuse to the outermost layer liquid surface and evaporate to thereby cause unevenness in coating before the liquid film is deposited on the web and gels thereon, even if the content of the low boiling point solvent in the outermost layer is zero.
Such uneven coating can be reduced by appropriately selecting the type and content of the surface-active agent in the outermost layer. This is because such uneven coating is caused by the uneven surface tension distribution produced on the liquid surface. Such surface tension distribution is apt to be produced particularly in the case where the liquid surface is expanded. Therefore, uneven coating can be reduced by appropriately selecting the type and content of the surface-active agent so as to relieve the surface tension when the liquid surface is expanded.